1. Field of the Invention
This invention relates to metallization processes for use in making devices such as semiconductor devices, and to devices formed using those metallization processes.
2. Related Art
Formation of a metal layer is a common step in the formation of some devices, such as, for example, semiconductor devices. In particular, a metal layer can be formed so as to fill in vias or cover steps formed during fabrication of a semiconductor device. The formation of a metal layer over vias having a high aspect ratio (i.e., ratio of the depth of the via to the width or diameter of the via) or steps having a relatively large height has been subject to several problems, such as cusping and voiding.
In one previous method of forming a metal layer on a semiconductor wafer, the metal layer is formed using a two step process. In the first step, a relatively thick portion of the metal layer is deposited while the semiconductor wafer is held at a relatively cold temperature (i.e., preferably less than or equal to 200xc2x0 C. ). The thickness of this portion must be adequate, in view of relevant process parameters (e.g., the geometry being metallized and the metal being used), to ensure that adequate metal is present to avoid the formation of voids during the metal reflow that occurs during the second step. For example, when the metal is an aluminum alloy, this thick portion preferably has a thickness equal to about 50% to 75% of the total thickness of the metal layer to be formed. Further, this portion is preferably deposited at a rate greater than about 150 xc3x85/sec. In the second step, the remainder of the metal is deposited while the semiconductor wafer is held at a relatively high temperature (e.g., when the metal is an aluminum alloy, about 400xc2x0 C. to about 500xc2x0 C. ) that allows the deposited metal to reflow through grain growth, recrystallization and bulk diffusion. The rate of deposition of the aluminum in the second step is preferably slower than that during the first step, but is preferably greater than about 50 xc3x85/sec., and more preferably between about 100 xc3x85/sec. and about 200 xc3x85/sec. Further, the deposition rate can be increased during the second step to increase the process throughput. However, this method does not minimize the number of defects formed in the metal layer (such as result from cusping and/or voiding, for example) as much as desired.
In another previous method of forming a metal layer on a semiconductor wafer, the metal layer is also formed using a two step process including a first, cold deposition step followed by a second, hot deposition step. However, in this method, a relatively thin portion of the metal layer (e.g., 25% of the overall thickness) is deposited while the semiconductor wafer is held at the cold temperature, while a relatively large portion of the metal layer (e.g., 75% of the overall thickness) is deposited while the semiconductor wafer is held at the hot temperature. When the metal is an aluminum alloy, the wafer can be held at a temperature of about 200xc2x0 C. for a period of about 10 seconds during the cold deposition step. During the hot deposition step, a heated gas (typically argon) is flowed against the backside of the wafer to heat the wafer and the deposited metal. The wafer can be heated to a temperature of about 375xc2x0 C. to about 500xc2x0 C. For the illustrative temperatures given, the wafer is typically held at that temperature for about 3-5 minutes. However, the heated gas flow is kept relatively low (e.g., less than about 15 sccm and typically in the range between about 10 sccm and about 15 sccm) so that the pressure within the process chamber can be kept low (e.g., less than about 2 mtorr). Since the heated gas flow is kept relatively low, the wafer is not heated as fast as is desirable to minimize the number of defects formed (e.g., by cusping and/or voiding) in the metal layer. Increasing the temperature of the heated gas has been tried as a means to improve this method; however, the increased gas temperature causes the steady state temperature of the wafer during the hot deposition step to increase, thus increasing the likelihood of damaging the wafer (in particular, metallization that has been previously formed on the wafer). Causing the heated gas to impinge on the wafer at multiple locations has also been tried; however, while this can cause the distribution of defects to be more evenly spread throughout the metal layer, it does not adequately reduce the overall number of defects.
The invention enables a layer of metal to be formed on a substrate with few or no voids formed in the layer. According to the invention, a layer of metal can be formed on a substrate using a cold deposition step followed by a hot deposition step. The cold deposition step need only be performed for a time sufficient to deposit metal over the entire surface on which the metal layer is to be formed. In the hot deposition step, further metal may be deposited while the substrate is rapidly heated to a target temperature. In particular, the invention enables the substrate to be heated more rapidly than has been the case in previous methods for depositing a metal layer using a cold deposition step followed by a hot deposition step. The rapid heating of the substrate results in the rapid heating of the metal deposited on the substrate. Heating this metal quickly causes the metal atoms to become mobile very quickly; in particular, the mobility of the most recently deposited metal atoms (which are typically furthest from the site of heat application) is enhanced. As a result, the deposited metal is far less susceptible to cusping and voiding than has been the case with previous methods for depositing a metal layer on a substrate. The rapid heating of the substrate can be accomplished by, for example, flowing a heated gas against the substrate at a flow rate that is higher than heretofore thought feasible.
The invention provides several advantages over previous methods of forming a metal layer. First, the invention enables a hot deposition step to be completed in a shorter period of time than has been the case in previous similar methods, thus providing increased throughput. Additionally, the invention may produce metal layers having few or no voids and, in particular, fewer voids than produced by previous methods. In particular, the invention can be used to reliably (i.e., so that 100% step coverage is achieved) fill tapered vias having an aspect ratio greater than 1:1, particularly when the via depth is about 0.5 micrometers or less. Further, the invention enables these advantages to be accomplished without increasing the temperature to which the substrate is heated, thus avoiding the increased potential for damage to the substrate and/or previously deposited or formed layers, lines or other structures associated with the use of higher temperatures.
In one embodiment of the invention, a method of forming a layer of metal on a surface of a substrate includes the steps of depositing a first amount of the metal on the substrate surface, then depositing a second amount of metal on the first amount of metal while heating the substrate from a cold temperature to about 95% of a target hot temperature at an average rate that is greater than or equal to about 10xc2x0 C. /sec., more preferably greater than or equal to about 15xc2x0 C. /sec., and most preferably greater than or equal to about 25xc2x0 C. /sec. The deposition of the first amount of metal need only be performed long enough to ensure that the metal is deposited to cover the substrate surface. The deposition of the second amount of metal can occur for long enough to complete the formation of the metal layer. Alternatively, the heating can be discontinued before the metal layer is complete and the remaining amount of metal deposited without application of heat (e.g., as the substrate cools). Heating the substrate quickly causes the atoms of the deposited metal to become mobile very quickly (for example, increases the mobility of the atoms enough to help the deposited atoms move and/or migrate after they are deposited on the substrate). As a result, the deposited metal is less susceptible to cusping and voiding than has been the case with previous methods for depositing a metal layer on a substrate. Moreover, the rapid heating enables such deposition to be accomplished in a shorter time period than has previously been possible, increasing throughput.
In another embodiment of the invention, a method of forming a layer of metal of a predetermined thickness on a first surface of a substrate includes the steps of depositing a first amount of the metal on the substrate surface, then depositing a second amount of metal on the first amount while flowing a heated gas against the substrate at a gas flow rate that is greater than or equal to about 15 sccm, more preferably greater than or equal to about 20 sccm, and most preferably greater than or equal to about 30 sccm. As in the previous embodiment, the deposition of the first amount of metal need only be performed long enough to ensure that the metal is deposited to cover the substrate surface. The gas can be, for example, argon or other inert gas. (The particular flow rate may depend to some degree on the gas used.) The deposited metal can be any appropriate metal, such as, for example, aluminum. Impinging the heating gas on the substrate at such high flow rates enables heat to be transferred to the substrate more quickly, thus enabling the substrate temperature to be increased rapidly, with attendant benefits, as discussed above. Further, the use of such high flow rates enables such rapid heating to be accomplished without raising the temperature of the heating gas, thereby avoiding the increased possibility of damaging the substrate and/or structures formed thereon associated with the use of higher gas temperatures.
In yet another embodiment of the invention, a substrate having first and second opposing surfaces is positioned within a process chamber and a method of forming a layer of metal on the substrate includes the steps of flowing a first gas into the process chamber at a location proximate to the first surface, then, after a predetermined amount of time, flowing a second gas into the process chamber so that the second gas, which is heated, flows against the second surface of the substrate, thereby causing the temperature of the substrate to increase. The first and second gases can be, for example, argon or other inert gas. The first gas interacts with a source of the metal in the process chamber to cause metal from the metal source to be deposited on the first surface of the substrate. The predetermined amount of time before flowing the second gas into the process chamber is sufficiently long to ensure that metal is deposited to cover the first substrate surface. When the second gas is flowed, the flow rates of the first and second gases are controlled so that the differential pressure across the substrate is not sufficient to cause the substrate to experience mechanical failure. This can be accomplished by controlling the ratio of the flow rate of the first gas to the flow rate of the second gas to be greater than or equal to about 2 and less than or equal to about 4, or, in a further embodiment, greater than or equal to about 2.5 and less than or equal to about 3. The flow rate of the first gas can be, for example, greater than or equal to about 40 sccm, more preferably greater than or equal to about 50 sccm, and most preferably greater than or equal to about 80 sccm. The flow rate of the second gas can be, for example, greater than or equal to about 15 sccm, more preferably greater than or equal to about 20 sccm, and most preferably greater than or equal to about 30 sccm. The pressure within the process chamber can also be controlled to be greater than or equal to about 2 mtorr. Such control of the gas flow rates enables a relatively high flow rate to be used for the second gas, thus enabling the temperature of the substrate to be increased more rapidly than previously possible, as discussed above.